Some switching circuits, such as converters or switching regulators, may be used to generate both positive and negative output voltages. For example, the LT®3471 dual switching regulator manufactured by Linear Technology Corporation, the assignee of the present application, includes two channels that may be configured as two boost converters, a boost converter and an inverter, or two inverters. Accordingly, each channel may be configured into a non-inverting boost regulator to produce positive output voltage or into an inverter to produce negative output voltage.
To support both positive and negative output voltages, this IC requires positive feedback pin FBP and negative feedback pin FBN for each channel and a reference pin Vref that may be shared by the channels. As illustrated in FIG. 1, the positive and negative feedback pins FBP and FBN are respectively connected to non-inverting and inverting inputs of an error amplifier EA that produces control voltage Vc at its output. Positive or negative output voltage Vout+ or Vout− is sensed using an external resistor divider composed of resistors R1 and R2. For the positive output, FBP is connected to the reference voltage pin Vref, FBN is connected between R1 and R2. For the negative output, FBN is tied to ground and FBP is connected between R1 and R2. If a negative output is sensed, a change in phase is provided.
Another example is a feedback scheme used in the LT®1370 switching regulator manufactured by Linear Technology Corporation, the assignee of the present application. This regulator can regulate positive or negative output voltage. As illustrated in FIG. 2, it has positive and negative feedback pins FB and NFB respectively provided for positive and negative output voltage sensing. Also, it has negative and positive error amplifiers EA1 and EA2, respectively. NFB is connected to the inverting input of the negative error amplifier EA1. The non-inverting input of EA1 is tied to ground. Via diode D, the output of EA1 is coupled to FB connected to the inverting input of EA2. The non-inverting input of EA2 is supplied with a 1.25V reference voltage.
Positive or negative output voltage Vout+ or Vout− is sensed using an external resistor divider composed of resistors R1 and R2. For positive outputs, NFB is shorted to FB which is regulated to 1.25V by the regulator loop. The output of EA1 is driven low, so the diode D is off. For negative outputs, EA1 drives FB to 1.25V so that NFB is regulated to 0V by the regulator loop.
To reduce the number of external pins provided in a chip for controlling a switching circuit capable of producing positive and negative output voltages, it would be desirable to develop a feedback scheme using a single pin for sensing both positive and negative output voltages.